An important method for the production of liquid cryogen, such as, for example, liquid nitrogen, comprises compression of gas, liquefaction, constant enthalpy expansion, and recovery. The constant enthalpy expansion, althouqh enabling the use of relatively inexpensive equipment, results in a thermodynamic inefficiency which increases energy costs.
It is an object of this invention to provide a liquefaction process which can operate with increased thermodynamic efficiency over heretofore available liquefaction processes.